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Community and Q&A

Crawl space liner for the dirt floor or closed-cell spray foam?

AWSEDRFT | Posted in Energy Efficiency and Durability on

I live in upstate NY and I have a 4 1/2′ high crawl space under my house with a dirt floor. I was told to insulate the walls with 3″ of closed spray foam and use a crawl space liner on the floor. Another contractor said I could use 2″ of closed spray foam on the walls and floor. Yet, another said 3″ should be used. Which is correct?
Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Most crawl space remediation contractors install insulation (for example, closed-cell spray foam) on the walls, but not on the floor. Usually, a heavy plastic liner on the floor is all you need.

    That said, installing closed-cell spray foam on the floor of your crawl space will reduce heat loss through the floor. The reason this work is rarely done is that the energy savings associated with installing spray foam on a crawl space floor aren't enough to make the measure cost-effective. (In other words, the cost is high and the returns are low.)

    There's another wrinkle: closed-cell spray foam is flammable, and the safest job includes some type of ignition barrier on the foam. Ideally, the spray foam on the floor would be covered by a concrete rat slab. That's expensive (but it would result in an excellent job).

    You live in Climate Zone 6, and the 2012 IRC calls for a minimum of R-15 on crawl space walls. Two inches of closed-cell spray foam is about R-12 -- maybe R-13 on a good day -- but not R-15. I'd go with 3 inches, not 2 inches, of closed-cell spray foam on the walls.

    Here is a link to an article with more information on this type of work: Building an Unvented Crawl Space.

  2. AWSEDRFT | | #2

    Thank you Martin for your answer. Seeing closed cell spray foam is flammable, should I be concerned with using it on the walls as well? Where does radon factor into this? How much should I expect to pay for a 1000' square plastic liner and is there a particular type of liner? Thanks again for your advice.

  3. GBA Editor
    Martin Holladay | | #3

    I can't comment on prices. If you are hiring a contractor, the price will be whatever local contractors are charging. If you are doing the work yourself, ordinary 6-mil poly would be the cheapest way to go. Thicker and tougher vinyl liners are available, but 6-mil poly will work if it doesn't get a lot of traffic.

    Contact your local building department to verify local code requirements for ignition barriers over spray foam in crawl spaces. Some jurisdictions insist on an ignition barrier (usually an intumescent coating), while others don't enforce the code or don't think a barrier is necessary. You can also ask the spray foam contractors. More info here: Thermal Barriers and Ignition Barriers for Spray Foam.

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